A considerable body of evidence has demonstrated an important role for cholinergic transmission in cognition, memory and behavioral state control. Like other amnestic or dementia-related disorders, diencephalic amnesia is accompanied by cholinergic dysfunction. Using an animal model of diencephalic amnesia we have documented cholinergic loss in the medial septum/diagonal band (MS/DB) and hypo-cholinergic output in the hippocampus that correlates with behavioral impairment, which can be alleviated by cholinomimetic drugs. These results can be attributed to 2 potentially orthogonal, mechanisms: (1) Cholinergic cell loss in the MS/DB region causes hypocholinergic output in the hippocampus during learning;(2) Lesions to thalamic and hypothalamic nuclei degrade neural activation in limbic regions and this is reflected in impaired ACh output during cognitive processing. The proposed research uses 2 rodent models of diencephalic amnesia to conduct a systems level analysis of the relationships between neuroantomical, neurochemical and behavioral dysfunctions seen in diencephalic amnesia. Aims: Using animal models we will: (A) Apply stereological microscopy techniques, in combination with immunocytochemistry, to fully document cholinergic cell loss in several important ascending cholinergic pathways;(B) Assess functional acetylcholine disruption by a novel application of in-vivo microdialysis/HPLC to assess ACh efflux during cognitive processing on a range of tasks and brain regions (hippocampus, amygdala, and dorsal striatum) connected to nuclei damaged in diencephalic amnesia;(C) Test whether hippocampal or septal administration of drugs that increase brain ACh levels will differentially lead to recovery of learning/memory function;(D) Map the functional diversity of diencephalic nuclei using discrete neurotoxin-induced lesions to determine if such lesions cause decreased ACh output in key memory structures. Significance: The neural mechanisms of diencephalic amnesia remain undetermined. Using in-vivo microdialysis on-line during cognitive testing in animal models of diencephalic amnesia is novel and will enhance our understanding of the interdependence between diencephalic and other limbic structures. Such experiments are critical to understanding the role of acetylcholine dysfunction in amnesia and thus the development of pharmacotherapeutics.